Here's an interesting piece of interior design. Challenged by a client to refurbish a tiny studio apartment in Paris, architect Betillon/Dorval‐Bory was asked to pay special attention to lighting due to the limited daylight available in some parts of the apartment. Its response was unusual, to say the least. High-performance lighting was installed at one end and, in one sense, deliberately awful lighting at the other. Named Appartement Spectral, the design breaks every rule in the book, yet the effect is striking – and it's all down to street lighting.

The only artificial lighting in the apartment is fixed to a small partition which separates the kitchen and living space from the sleeping areas (a bunk of a crawl space) and bathroom (a shower underneath). On the kitchen side are seven sleek and modern fluorescent tubes which are high performance in practically every sense. On the other are a pair of low-pressure sodium lamps, which are the kind you'd ordinarily find in street lighting.

In one way, low-pressure sodium lamps (sometimes referred to as SOX lamps for the layer of indium tin oxide infra-red film they're generally coated in) are terrific. Though LEDs are making rapid headway, low-pressure sodium lamps are extremely efficient, approaching 200 lumens/watt. But as anyone who has walked along their street after dark will know, low-pressure sodium lamps cannot render colors accurately. Everything beneath them appears to be the same yellow-brown color of the light itself. This is because SOX lamps emit light at a single wavelength of about 590 nm.

Lighting nerds will be familiar with the color rendering index of a light source, which is a quantitative scale which tops out at 100 for sources which perfectly render color (a group limited to black body radiators). The complexities of the math mean that negative CRIs are theoretically possible, and though sources differ enormously on the CRI of low pressure sodium lamps, if the figure strays far from zero, it's in the wrong direction. Low pressure sodium light is literally monochromatic, though not so much black and white as black and amber.

Though it may sound like madness to use low-pressure sodium lamps in a designer apartment, this was completely deliberate. In fact, the division of space within the apartment was on the grounds of "spectral needs." Sleeping and showering have been deemed monochromatic pastimes, though if the client is intending to read in bed, picture books and glossy magazines may be best avoided. On the other side of the partition the fluorescent light scores a CRI of above 90, thanks to the mixture of phosphors used to coat superior quality lamps to emit light in different wavelengths. Cooking is one activity during which you want colors to appear as they should.

Efficiency aside, there are no obvious advantages to using low-pressure sodium lamps except as a bold design statement. In fact they pose certain practical difficulties. Low-pressure sodium lamps take 10 minutes to start, so any sort of automation or smart control, other than a timer, is impractical (if not impossible). Interior lighting is not something people are generally used to planning around, and it's for this reason that most gas-discharge lamp sources aren't even considered for use indoors.

All of which said, I think this is very clever design, and not just for the lighting. People, though perhaps not hygiene obsessives, on the lookout for space-saving design tips may like to pay attention to the stairs up to the kitchen counter, which doubles as a half landing. It's all tremendous fun, but whether you'd want to live there perhaps depends on whether you prize individualism above practicality.

Update 05.13.2013: This article has been amended. It previously stated that low-pressure sodium lamps take time to "restrike" having recently been turned off. This is not the case. Thanks to the reader that pointed this out.

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